1. Field of the Invention
The present invention relates to an image forming apparatus, which is applicable to, e.g., a laser beam printer and, more particularly, to a color image forming apparatus for sequentially transferring a plurality of color visible images sequentially formed on an image carrier onto a transfer medium so as to overlap each other.
2. Related Background Art
In recent years, color printers have become available, and are utilized as various expression means for users. In particular, color page printers have received a lot of attention due to their quiet, and high-quality, high-speed printing characteristics.
As the characteristic feature of a color laser beam printer as a kind of color page printers, a process for scanning a light beam on a photosensitive body in the main scanning direction to form a first latent image, performing first development, and thereafter, transferring the first latent image onto a recording medium such as a recording sheet on a transfer carrier, is executed as a first process, and a multi-color image is recorded by subsequently executing second, third, and fourth processes in different colors.
A color printer will be described below with reference to FIGS. 1 and 2.
FIG. 1 is a sectional view of the printer. In FIG. 1, a mechanism section A of the color laser beam printer includes a photosensitive drum 1, a charger 4, a semiconductor laser 5, a scanner motor 6, a polygon mirror 7 rotated by the scanner motor, a lens 8, and a mirror 9. The semiconductor laser 5 outputs a light beam L, which is ON/OFF-modulated according to an image signal S10 (to be referred to as a VDO signal hereinafter).
The light beam emitted from the semiconductor laser 5 is scanned by the polygon mirror 7, and is then guided to the photosensitive drum 1 through the lens 8 and the mirror 9.
A developing unit 3Y develops a latent image formed in the first process so as to form a yellow toner image as a first toner image. A developing unit 3M develops a latent image formed in the second process so as to form a magenta toner image as a second toner image. A developing unit 3C develops a latent image formed in the third process so as to form a cyan toner image as a third toner image. A developing unit 3BK develops a latent image formed in the fourth process so as to form a black toner image as a fourth toner image. A paper cassette 15 stores recording sheets P. The recording sheets P are fed one by one from the cassette by a pickup roller 14.
A transfer drum 16 is constituted by a support member 27 and a film 28. A cleaner 10 removes a non-transferred toner upon completion of each transfer process. The mechanism section A also includes a fixing unit 13, a delivery tray 19, a charger 11, and a peeling pawl 12. A detector 23 detects the leading edge of a sheet on the transfer drum 16, and outputs a vertical sync signal S4. A detector 17 is arranged on the scanning axis of the light beam, and outputs a beam detect signal (to be referred to as a BD signal hereinafter) S6 serving as a horizontal sync signal upon incidence of the light beam L.
The operation of this arrangement will be described below. The photosensitive drum 1 is primarily electrified in a predetermined polarity at a predetermined voltage by the charger 4. When the leading edge of the sheet is detected by the sheet leading edge detector 23, the light beam L modulated by the VDO signal S10 scans and exposes the photosensitive drum 1 in synchronism with the BD signal S6, thereby forming a first electrostatic latent image. The first electrostatic latent image is developed by the developing unit 3Y, thus forming a yellow first toner image on the photosensitive drum 1.
On the other hand, the recording sheet P is fed at a predetermined timing. Immediately before the leading edge of the sheet reaches a transfer start position, a predetermined transfer bias voltage in a polarity opposite to that of the toner is applied to the transfer drum 16, so that the recording sheet P is electrostatically attracted on the surface of the transfer drum 16, and the first toner image transferred onto the recording sheet P.
A second electrostatic latent image is formed on the photosensitive drum by the light beam L, and thereafter, is developed by the developing unit 3M, thereby forming a magenta toner image on the photosensitive drum 1. The second toner image is transferred onto the recording sheet P at a position of the first toner image previously transferred onto the recording sheet P in synchronism with the recording sheet. Similarly, third and fourth electrostatic latent images are formed on the photosensitive drum 1, and cyan and black toner images are transferred in synchronism with the recording sheet P, thus forming toner images of four colors on the recording sheet P. In this manner, the VDO signals S10 for one page are sequentially output to the semiconductor laser 5 in each process.
When the leading edge of the recording sheet P, on which the toner images of four colors have been transferred, approaches the peeling pawl, the peeling pawl 12 is moved close to the transfer drum 16, so that its distal end is brought into contact with the surface of the transfer drum 16, thereby peeling the recording sheet P from the transfer drum 16. The distal end of the peeling pawl 12 is kept in contact with the surface of the transfer drum 16 until the trailing edge of the recording sheet P is peeled from the transfer drum 16. Thereafter, the peeling pawl 12 is returned to its home position. The charger 11 discharges an electrified charge on the recording sheet P to facilitate peeling of the recording sheet P by the peeling pawl 12, and to eliminate air discharge upon peeling.
FIG. 2 is a block diagram of a laser beam scanning apparatus in a controller of the above-mentioned printer.
The same reference numerals in FIG. 2 denote elements having the same states as in FIG. 1. The image forming operation of the printer apparatus will be described below with reference to FIG. 2.
The scanning apparatus shown in FIG. 2 includes a reference oscillator 20, a frequency divider 21 for frequency-dividing a reference clock S1 output from the reference oscillator with a predetermined value, and a motor control circuit 25 for controlling uniform rotation of the scanner motor 6 to follow a frequency-divided clock S2 output from the frequency divider 21.
The motor control circuit 25 incorporates a known phase control circuit (not shown), and rotates the scanner motor 6 at a constant speed, so that a phase difference between a feedback signal S7 from the scanner motor 6 and the frequency-divided clock S2 is equal to a predetermined phase difference. The photosensitive drum 1 and the transfer drum 16 are rotated at a constant speed by a drive motor 30. The leading edge of the recording sheet P on the transfer drum 16 is detected by the detector 23, and the detector 23 outputs a vertical sync signal (to be referred to as a VSYNC signal hereinafter) S4 to an image generator 29. The leading edge of an image of each color is regulated by the VSYNC signal S4. The VD0 signals S10 are sequentially sent to the laser 5 in synchronism with BD signals S6 after the VSYNC signal S4.
However, in the above-mentioned technique, toner images of four colors are respectively developed and formed on an image carrier in units of colors, and are caused to overlap each other on a transfer carrier, thereby forming a multi-color toner image. For this reason, an HSYNC signal is shifted from the VSYNC signal (t3 to t6), as shown in FIG. 3, and a finally fixed color image on a recording sheet suffers from color misregistration in units of color components, as shown in FIG. 4.
As a method of preventing the color misregistration, rotational control precision of drive systems such as a scanning optical system, a photosensitive/transfer drum system, and the like must be individually improved. However, when the scanner motor comprises a high-precision motor using an air bearing, and the transfer drum employs a flywheel, this results in an increase in size of the printer, and an increase in manufacturing cost. Even when rotational precision is improved, the image start position of the first color cannot often coincide with those of the second and third colors due to the relationship between rotational speeds of the scanning optical system and the photosensitive/transfer drum system.
For example, when a printing operation is performed at a density of 400 dpi, if the transfer drum has an outer diameter of 120 mm, 5936.868 (=120.times..tau..div.(25.4.div.400)) lines can be drawn on the outer circumferential surface of the drum. When an image of the second color is printed, misregistration of 0.868 lines, i.e., 0.868.times.(25.4.div.400)=55.1 .mu.m, occurs at the beginning of the image. Since an image of the third color is further shifted by 55.1 .mu.m, it is shifted from the image of the first color by 110.2 .mu.m. Since an allowable limit of color misregistration is about 20 .mu.m in a visual sense, the color misregistration is conspicuous in the above-mentioned case.